1. Field of the Invention
The present invention relates to a magnetic detector for detecting changes in a magnetic field caused by movement of a moving member of magnetic material, and more particularly to a magnetic detector with a function of detecting alternately projected and recessed portions of a moving member of magnetic material in a power-on condition (referred to as a power-on function hereinafter).
2. Description of the Related Art
There is a known technique for detecting changes in a magnetic field wherein electrodes are formed at both ends of a magnetic field sensitive surface of magnetic field sensing devices (magnetoresistive devices are used in the description here) to construct a bridge; a constant-voltage and constant-current power supply is connected between the two opposing electrodes of the bridge to convert changes in the resistance value of the magnetic field sensing device into voltage changes, and changes in the magnetic field acting on the magnetic field sensing devices are detected on the basis of the voltage changes.
FIG. 9 is a circuit diagram showing an arrangement of a processing circuit in such a sensor using ordinary magnetic field sensing devices.
In FIG. 9, a Wheatstone bridge circuit 1 is made up of magnetic field sensing devices or resistances RA, RB, RC and RD given including at least one or more magnetic field sensing devices. Input terminals of a differential amplification circuit 2 are connected to a middle junction 4 between RA, RB and a middle junction 5 between RC, RD. A middle junction between RA, RD is connected to a power source terminal vcc, and a middle junction between RB, RC is connected to a ground GND. A differential amplification output 8 from the differential amplification circuit 2 is supplied to a comparison circuit 3 in a next stage.
In the above circuit, resistance values of the magnetic field sensing devices are changed upon changes in the magnetic field applied to resistances RA, RB, and a voltage at the middle junction 4 between RA, RB is changed corresponding to such changes in the applied magnetic field. The voltage across the middle junctions 4 and 5 is amplified by the differential amplification circuit 2, and a final output 9 having a level of "0" or "1" is obtained from the comparison circuit 3.
FIG. 10 is a schematic view showing a construction of a conventional magnetic detector.
In FIG. 10, the conventional magnetic detector comprises a rotary member of magnetic material 10 having a shape capable of changing a magnetic field, a magnetic field sensing device 11, magnetic field sensing elements 11a, 11b, a magnet 13, and a rotary shaft 12. When the rotary shaft 12 rotates, the rotary member of magnetic material 10 is also rotated in synchronous relation therewith.
The pitch center of the magnetic field sensing elements (resistances) 11a, 11b of the magnetic field sensing device 11 is arranged to be offset from the center of the magnet 13 by a predetermined amount.
In the above magnetic detector, upon rotation of the rotary member of magnetic material 10, the magnetic field applied to the resistances 11a, 11b of the magnetic field sensing device 11 is changed, and as shown in by way of example FIG. 11, the differential amplification output 8 is changed corresponding to the shape of the rotary member of magnetic material 10. As a result, a signal representing the final output 9 corresponding to the shape of the rotary member of magnetic material 10 can be obtained with the circuit shown in FIG. 9.
However, the magnetic circuit arrangement used in the conventional detector has had the following problems.
When the bridge is made up of magnetic field sensing devices and fixed resistances, a difference between their temperature coefficients. Also, when the bridge is made up of magnetic field sensing devices with a plurality of elements, a difference in the temperature coefficients occurs due to a difference in the magnetic field applied to the elements. Since there is such a difference in temperature coefficients, as shown in FIG. 12, the differential amplification output 8 (ROOM) at room temperature and the differential amplification output 8 (HOT) at high temperature exhibit temperature characteristics depending on changes in the applied magnetic field. This gives rise to a large deviation in the edge detecting accuracy of the alternately projected and recessed portions of the rotary member of magnetic material.